ASSESSMENT OF THE IMPACT OF DECAY CORRECTION IN THE DOSE-TO- CURIE METHOD FOR LONG-TERM STORED RADIOACTIVE WASTE DRUMS

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1 ASSESSMENT OF THE IMPACT OF DECAY CORRECTION IN THE DOSE-TO- CURIE METHOD FOR LONG-TERM STORED RADIOACTIVE WASTE DRUMS K. H. Hwang, S. C. Lee, S. H. Kang, K. J. Lee Korea Advanced Insttute of Scence and Technology (KAIST) T. W. Km, K. D. Km, M. J. Song Nuclear Envronment Technology Insttute (NETEC) ABSTRACT Regulatons and gudelnes requre the detaled nformaton about the characterstcs of radoactve waste drums. Therefore t s mportant to know the accurate radonuclde nventory of radoactve waste. However, evaluaton of radonuclde concentratons n the drummed radoactve waste s dffcult. The gamma scannng method can be used for the assessment of gamma emttng radonucldes. In Korea, a radoactve waste assay system has been operated at Korean nuclear power plant (KORI ste) snce 996 and the consoldated scalng factor (SF) concept has played a domnant role n the determnaton of dffcult-tomeasurable (DTM) radonuclde concentraton. However, f the radoactvty n the radwaste drum s extremely low or hgh, the radonuclde actvty of the drum cannot be evaluated properly. Gamma scannng s also tme-consumng and mposes an economc burden to the utltes. In order to overcome these dffcultes, dose-to-cure converson (DTC) method, whch s supplementary to the gamma scannng method, has been used to assess the radonuclde actvty of radwaste drums n KORI ste. Snce then, a research for the mprovement of exstng methodology has been performed n A further detaled research has been progressed for more relable assessment and contnuous renewal of data snce The scope of research s expanded and planned such as followng. Frst, assay target nucldes (22 radonucldes) and NPPs (all of the Korean NPPs ncludng Pressured Heavy Water Reactors) are ncreased. In addton, more relable samplng (actual waste samples) and measurement technques have been performed and utlzed. Conformaton of correlaton pars based on Korean analyzed data wll be provded. Through these amendments, accuracy and relablty of assessment wll be mproved. There are long-term stored LLW drums more than 60,000 n Korea NPPs. Some of them cannot be analyzed by a gamma scannng method. It s, thus, necessary to apply the dose-tocure method for these LLW drums. Due to the dffculty of broad radochemcal analyss for long-term stored drums, current radochemcal analyzed database wll be used on the

2 assumpton that the trend (SF and fractonal abundance of gamma emtter) s smlar when t was generated. Therefore, SF and fracton abundance need to be corrected, whch s based on the hstorcal records. In ths study, the mpact of decay correcton n the actvty determnaton s performed based on the assumed hstorcal records. From the case study, t s concluded that DTC method needs to nvolve the decay correcton, whch s based on the hstorcal record. Therefore, decay correcton algorthm s reflected to the DTC method. INTRODUCTION In Korea, the Enforcement Decree of the Korean Atomc Energy Act (Artcles 88) requres the detaled nformaton about the radoactve waste package. The measurement of concentraton and total actvty of radonuclde contaned n radwaste drum s very mportant for the accurate and effcent management of radoactve waste n NPPs. An establshed waste characterzaton program n KORI ste measures the concentraton of gamma-emttng radonucldes drectly usng the gamma assay system and/or ndrectly usng the dose-to-cure converson method. The concentraton of other relevant radonucldes s estmated ndrectly by relatng DTM radonucldes to other Easy-To-Measurable (ETM) radonucldes. SFs are generated by use of sample data that are gathered from the radochemcal analyses of waste samples collected from the dfferent waste stream. The determnaton of actvty s conducted by radonuclde assay system and SF method. However, some problems are remaned. Some mportant radonucldes are not ncluded n ths program. Other PWRs except NPPs of KORI ste and PHWRs are not consdered. It also needs to more number of samplngs and relable samplng procedures for the mprovement of relablty. For that reason, t s n progress to establsh an assay system usng more relable methodology for updatng the performance of Korean nuclear waste management. Korean hydro & nuclear power co. Ltd. (KHNP) organzes the overall project wth partal cooperaton wth Korea power engneerng company Inc. (KOPEC) [tomographc gamma Scanner system], Korea atomc energy research nsttute (KAERI) [radochemcal analyss of samples] and Korea advanced nsttute of scence and technology (KAIST) [scalng factor evaluaton and dose-to-cure converson program]. In ths paper, the status of radonuclde actvty determnaton method n KORI ste and new system are brefly ntroduced. In addton, samplng analyzed data set s assessed for the determnaton of target gamma-emttng nucldes and fractonal abundance n DTC method. The mpact of decay correcton s also assessed from the vewpont of the varaton of fractonal abundance and actvty of each gamma emttng radonuclde n a typcal radwaste drum.

3 Status of Radonuclde Actvty Determnaton Method n Kor System and New System At the end of 993, Korea electrc power research nsttute (KEPRI) organzed the overall project to desgn and nstall the radoactve waste assay system wth partal cooperaton wth KAERI and KAIST [, 2]. Wth careful consderatons, KORI NPP was selected as a canddate ste for assay system. Radoactve waste assay system was nstalled and started operaton durng the md 996. In ths research, actvty determnaton was conducted by radoactve waste assay system for the key nucldes and SF method for DTM nucldes. A further detaled research has been progressed for more relable assessment and contnuous renewal of data snce The project of new radoactve waste assay system s started n 2003 and wll be fnshed at the end of Prncpal changes of new system compared to KORI system are assay target DTM radonucldes and NPPs. Comparson between KORI system and new system s summarzed n Table I. [3] Table I. Comparson of KORI System and New System KORI System New System Gamma assay method Target DTM radonuclde Target NPPs Waste types and representatve samplng SGS (Segmented Gamma Scannng) & DTC method H-3, C-4, Fe-55, Co-60, N-63, Sr-90, Nb-94, Tc-99, Cs-37 and gross alpha KORI ste A) Spent flter (RCS Letdown flter) B) Concentrates (Radwaste Evaporator Concentrate) C) Spent resn (Prmary Mxed Bed Resn) D) DAW (Dry Actve Waste) TGS (Tomographc Gamma Scalng) & DTC method H-3, C-4, Fe-55, N-59, N-63, Co-60, Sr-90, Nb-94, Tc-99, I-29, Cs-37, [Pu- 238, 239, 240, 24], Am-24, [Cm-242, 244] and gross alpha All Korean NPPs (6 PWRs and 4 PHWRs) A) Spent flter (RCS Letdown flter) B) Concentrates (Radwaste Evaporator Concentrate) C) Spent resn (Prmary Mxed Bed Resn) D) DAW (Dry Actve Waste) E) Sludge Analyss of Samplng Radochemcal Analyss Data Set The representatve samplng s n progress for the evaluaton of scalng factor and applcaton of DTC method. In ths study, cumulatve samplng data set s analyzed and t s focused on gamma radonuclde data set for the determnaton of fractonal abundance of gamma

4 emttng radonucldes. Informaton of samplng data set s summarzed n table II. It s nearly mpossble to obtan the relable representatve samplng for overall radwaste drums, especally, for the long-term stored drums. Therefore, fractonal abundances of gamma nucldes are derved from the current samplng data set and should be corrected by the use of the hstorcal record, whch s manly the tme perod for decay correcton. Table II. Samplng Data Set of Gamma Emttng Radonucldes Waste Type Detaled Classfcaton Samplng Ste (Number of Sample) Cotton (7) Kor (6) DAW (8) Paper (5) Ulchn (9) Vnyl (6) Yonggwang (3) Kor (2) Concentrates (5) - Ulchn (2) Yonggwang () Spent resn (5) Kor () Hgh level (2) Ulchn () Low level (3) Yonggwang (3) Spent flter () - Ulchn () Kor (2) Sludge (4) - Ulchn () Yonggwang () Lst of Nuclde Co-60 Cs-37 Ag-0m Ce-44 Co-57 Co-58 Cr-5 Cs-34 Fe-59 Mn-54 Nb-95 Sb-25 Zr-95 Establshment of the Decay Correcton Method n DTC Method and ts Applcaton DTC method estmates the concentraton of gamma-emttng radonucldes n a waste package based on the dose rate. The dose rate s measured at a specfed dstance and geometrcal poston from the waste package. Ths nformaton, combned wth the data on the relatve abundance of the gamma-emttng radonucldes, s used. The radonucldespecfc CFs are determned usng the MCNP-4C n ths study [4, 5]. The basc equatons are same as followng: C total = n = f D CF (Eq. ) C = Ctotal f (Eq. 2) where:

5 C total = Total actvty (mc) C = Radonuclde specfc actvty (mc) D = Average surface dose rate (mr/hr) f = Fractonal abundance of the th radonuclde CF = Radonuclde specfc dose-to-cure converson factor for a gven waste drum confguraton (mr/hr/mc) Decay correcton can be done from the hstorcal record. Modfed equatons are same as followng. f C ' = ' total n f = = e f n = λt e f ' λt D C = C f ' ' total CF (Eq. 3) (Eq. 4) (Eq. 5) where: C total, C = Modfed total actvty and radonuclde specfc actvty, respectvely (mc) f = Modfed fractonal abundance of the th radonuclde λ = Decay constant (sec - ) T = Decay correcton tme (= Measured tme Generaton tme) (sec) From the survey of status of long-term stored drum n Korean NPPs, t s concluded that there exst 22 dfferent radwaste drums, whch can be applcable to DTC method as a complement method for gamma assay system. They are classfed by the drum type, contaned radwaste type and soldfcaton process. To show the mpact of decay correcton n the determnaton of gamma actvty, a case study s conducted n a typcal drum. In ths case, concrete drum type II contanng the concentrate s selected. In Ulchn ste, concentrate bottom s soldfed usng the cement wthout the process of dryng and stored n concrete drum type II untl 998. Snce then, CWDS (concentrate waste dyng system) s operated. In ths case, cementsoldfed concentrate bottom s selected as a target of case study and summarzed n Table III. Drum modelng and related nput data are brefly ntroduced n Fgure 4.

6 Table III. Summary of Input Data for Radwaste and Drum Related Ste Ulchn Drum type Concrete drum type II (Inner volume : 350L) Radwaste Concentrate bottoms Soldfcaton Cement Generaton tme 88'~98' φ =08 (Concrete) 8 2 φ =77.8 (Concentrates + Cement) t=30 Inner metal drum: h=30 (Concrete) t=30 φ =40 Fg.. Modelng of Concrete Drum Type II n Ulchn NPP Results and Dscusson Pror to analyss of the mpact of decay correcton n DTC method, selecton of gamma radonucldes and determnaton of fractonal abundance are conducted for concentrate bottom n the data set. It s restrcted to the gamma radonuclde, whch ts average fractonal abundance s hgher than 0.0(%). Fractonal abundance and CF values are summarzed n Table IV. Table IV. Fractonal Abundance and CF Gamma Radonuclde Intal Fractonal Abundance (%) Converson Factor (mr/hr/mc) Co E-02 Cs E-04

7 Ag-0m E-03 Cs E-03 Mn E-03 Nb E-04 Co E-03 Decay correcton s conducted as a functon of assumed decay correcton tme (=Measured tme-generaton tme). Decay correcton tme s assumed from 6 to 6 yr, whch s correspondng to the generaton tme of radwaste from 988 to 998. Fractonal abundance s corrected to a current one, whch s based on the generaton tme of radwaste and shown n fgure 2. On the contrary to long-lved radonucldes, short-lved radonucldes, such as Co-58 and Nb-95, are decreased rapdly, a hghly short-lved radonucldes, such as Mn-54 and Ag- 0m, are decay out n a short tme. Therefore, actvtes of these short-lved radonucldes decrease rapdly and can be neglgble n a short perod of tme. Based on the corrected fractonal abundance, corrected actvty of each gamma radonuclde s drawn n Fgure 3. In ths fgure, t s assumed that average surface dose rate s 00 mr/hr. Corrected actvty as a functon of generaton year s shown n fgure 3. Actvty n long-term stored radwaste s composed of manly Co-60 and Cs-37. Especally, contrbuton of Cs-37 to the total actvty s ncreased as the stored tme goes on. Other radonucldes except the Co- 60 can be gnorable n the long-term stored drum. In fgure 4, actvty rato s shown between the corrected and the uncorrected actvty. Long-lved Cs-37 s predomnant n long-term stored radwaste drum, and ts actvty rato s ncreased as the decay correcton tme s ncreased. However, actvty rato of Co-60 s not changed sgnfcantly. Other relatvely short-lved radonucldes are decreased sgnfcantly as decay correcton tme ncreased.

8 80% 70% 60% 50% 40% 30% 20% 0% 0% (04') C orrected relatve abundance(% ) C s-37 C o-60 C s-34 M n-54 A g-0m C o-58 Nb-95 98' 97' 96' 95' 94' 93' 92' 9' 90' 89' 88' Decay correcto n tm e (y e a r o f g e n e ra to n ) (y r) Fg. 2. Corrected fractonal abundance as a functon of generaton year 3.E+04 C orrected actvty(m C ) 2.E+04 2.E+04.E+04 5.E+03 C s-37 C o-60 C s-34 M n-54 A g-0m C o-58 Nb-95 0.E+00 (04') ' 97' 96' 95' 94' 93' 92' 9' 90' 89' 88' Decay correcto n tm e (y e a r o f g e n e ra to n ) (y r) Fg. 3. Corrected actvty as a functon of generaton year

9 Actvty rato (C orrected/n otcorrected) 700% 600% 500% 400% 300% 200% 00% 0% (04') C s-37 C o-60 C s-34 M n-54 A g-0m C o-58 Nb-95 98' 97' 96' 95' 94' 93' 92' 9' 90' 89' 88' Decay correcto n tm e (y r) Fg. 4. Actvty rato between the corrected and uncorrected actvty CONCLUSIONS In ths paper, the mpact of decay correcton s assessed from a vewpont of the varaton of fractonal abundance and actvty for each gamma emttng radonuclde usng a typcal radwaste drum. Frst, the radoactve waste assay system n KORI ste and new system are brefly ntroduced and compared to each other. In the new system and program, target NPPs are expanded from KORI unt to other PWRs and PHWRs. The numbers of assay target radonucldes are also ncreased from 0 to 22. From the st accepted samplng data set, samplng data set s assessed and CF s evaluated usng the MCNP-4C code for the typcal long-term stored radwaste drum n Ulchn NPPs. Fractonal abundance and actvty of domnant gamma emttng radonucldes are evaluated. From the case study, t s concluded that decay correcton process s essentally for the accurate actvty evaluaton and should be used for the relable applcaton of DTC method. Decay correcton s also necessary for the preventon of an nordnate under-estmaton of key radonuclde actvty n a long-term stored radwaste drum. An addtonal and frequent samplng procedure s n progress to update samplng data set. As ths study goes on, conformaton of correlaton pars based on the Korean analyzed data wll be provded. Accuracy and representatveness of related parameters, such as SF, fractonal abundance and so on, wll be mproved. Through these progresses, more accurate and relable predcton of radonuclde nventory n radoactve waste based upon Korean sampleanalyzed data set wll be possble.

10 ACKNOWLEDGEMENT Ths work was performed by the fnancal support of Korean hydro & nuclear power Co. Ltd. (KHNP). REFERENCES. Jn Beak Park, Kun Ja Lee et al, Status of Operaton of Radonucldes Assay System n Korean Nuclear Power Plant, The 8th Internatonal Conference on Envronmental Management, Bruges, Belgum, Sep 30-Oct 4 200(200) 2. Kang, D. W. et al, Development of a Radoactve Waste Assay System, KEPRI-92N- J03, KEPRI, Daejeon, Korea (996) 3. Kun Ja Lee, et al, "Development of radonuclde concentraton determnaton program for radoactve waste drums by use of scalng factor and dose-cure converson factor", Interm report, NETEC, Daejeon Korean (2004) (not publshed) 4. Judth F. Bresmester, "MCNP-A General Monte Carlo N-Partcle Transport Code: Verson 4C", LA-3709-M (2000) 5. S. R. Gedeon "Bass for Dose Rate to Cure Assay Method", WHC-SD-WM-RPT-267, Westnghouse Hanford Company, Rchland, U.S. (995)